1. Field of the Invention
The present invention relates to a modulation control type of AC machine, and more particularly, to a modulation control type of AC machine which can remove drawbacks of a conventional AC machine to improve the characteristics of industrial facilities and has many application fields.
2. Description of Related Art
Conventional AC machines can be classified into AC motors and AC generators and the structures of AC machines are different from each other depending upon the types. Generally, the AC motor is employed for driving and controlling various types of industrial facilities and the AC generator is used for generation of power, using a prime mover. However, the AC machines differ from each other in the characteristics and purposes depending upon the types. Therefore, the type of the AC machine is to be selected based on a usage purpose. For instance, there are a drawback of rotation speed control in a synchronous motor and a drawback of change of rotation speed due to the load of a rotor in an induction motor. The problems of the conventional various types of AC machines will be described below.
The conventional various types of AC machines such as a synchronous AC machine, an induction AC machine and a selsyn motor have different structures, operation principle and characteristics from each other and have the respective inherent problems. The modulation control type of AC machine according to an aspect of the present invention has an indirect relation to the conventional various types of AC machines. The basic principle of the modulation control type of AC machine according to an aspect of the present invention can be explained by using the relation. However, in order to make clear the difference between the present invention and the conventional AC machines, the problems of the conventional AC machines will be described below.
A synchronous motor uses a rotary magnetic field which is constituted by a DC main magnetic flux which is proportional to the frequency of an AC power source and rotates with a synchronous rotation speed. Therefore, DC electromagnetic force generated by field windings through which DC field current is flown acts as synchronous torque for rotating a rotor with a synchronous rotation speed. In this case, however, the rotor has a delay angle in relation to the DC main magnetic flux. The synchronous motor requires an AC power source and a DC power source and this is a drawback on the structure. The rotation speed and synchronous torque are controlled by changing the AC power source frequency and the DC field current, respectively. For purpose of increasing the rotation speed, if the AC power source frequency is increased, the magnetizing current, DC main magnetic flux and synchronous torque are decreased. Accordingly, the voltage of AC power source needs to be increased for the purpose. Further, if the AC power source frequency is decreased for purpose of decreasing the rotation speed, the magnetizing current is increased to cause magnetic saturation, as contrary to the above case. Accordingly, the AC power voltage needs to be decreased. As a result, the size of synchronous motor becomes larger and larger for response to the change of AC power source voltage, resulting in increased weight and reduced efficiency.
In the synchronous motor, alternate magnetomotive force is generated which is proportional to the product of the number of turns of the stator winding for each value of phase and load current. However, the synchronous motor has no secondary circuit for canceling the AC magnetomotive force. Therefore, there is a drawback in armature reaction which generates reactance to prevent the load current. The armature reaction is the causes of distortion influencing to the DC main magnetic flux generated by the magnetizing current and demagnetizing action. If the alternate magnetomotive forces of respective phases are synthesized, DC magnetomotive force can be obtained. Further, if the load torque of the rotor is greater than the synchronous torque, the synchronous motor causes the step-out from the synchronization to stop rotation, resulting in loss of the synchronous torque. If the DC field current is increased in order to prevent the step-out, the polarization by the DC excitation causes magnetic saturation. Therefore, the AC power source increases the exciting current so that the power factor is decreased and the temperature is increased.
A synchronous generator is necessary to be equilibrium between the AC power source voltage and the generated voltage, power is supplied through the generated voltage and its phase. Accordingly, in order to keep the synchronous rotation speed, a speed control unit with precise control capability is applied to a prime mover. Since the power is generated based on a lead angle of the rotor, a voltage difference is necessary between the AC power source and the synchronous generator. In the power generation, the load current and magnetizing current are supplied from the synchronous generator to the AC power source. If the direction of voltage difference is inverted, the synchronous generator acts as a synchronous motor. If the generated power is changed, the load current and armature reaction change the generated voltage and the phase. There is a drawback that circulating current flows between synchronous generators operated in parallel so that turbulence of generated power is readily caused due to the synchronizing function.
In the induction motor, since primary winding corresponds to stator winding of the synchronous motor, the same rotary magnetic field as in the synchronous motor is generated in the stator. The relation of the primary winding and the secondary winding is similar to the electromagnetic coupling in a transformer and a load is connected to the secondary winding. Since the load current is supplied from the AC power source, the induction motor has a simple structure, compared to that of the synchronous motor. Since the load current flowing through the primary winding in each of the phases is proportional to that flowing through the secondary winding of the corresponding phase, the alternate magnetomotive force representative of the product of the number of turns in the primary winding and the load current flowing in the primary winding and that of representative of the product of the number of turns of the primary or secondary winding and the load current are equal to each other and cancelled. Therefore, there is no armature reaction and any reactance preventing the load current is removed.
Exciting current flows in each phase of the primary winding. If the components of reactive current as the magnetizing current are synthesized over all the phases, an equivalent DC magnetizing current is obtained which generates a DC main magnetic flux. When secondary reactive currents having the same phase relation as the magnetizing currents are synthesized over all the phases, an equivalent DC current is obtained similarly which corresponds to a DC field current of a synchronous motor. Thus, in the induction motor, the rotor is rotated based on the synchronous torque which is generated with the same principle as in the synchronous motor. Further, it indicates that the secondary reactive current is necessary for the rotation. Since the secondary active current in each phase has a phase difference of .pi./2 in relation to the magnetic flux generated by the primary winding in the phase, an average of AC electromagnetic forces which is proportional to the product of the secondary active current and the number of turns of the secondary winding is zero. Therefore, the secondary active current has no relation to the synchronous torque.
In the induction motor, the rotation speed changes due to slip of the rotor so that the secondary voltage and the frequency decrease. Equivalent DC current reduces because of the reduction of secondary voltage and the rotation speed is decreased from the synchronous rotation speed because of the slip. Actually, since the rotation of the rotor itself is added to the rotation of a DC magnetic flux in which the slip frequency of the rotor is taken into account, the rotor rotates in the same direction as the rotation direction of the DC magnetic flux in synchronous with the DC main magnetic flux by the stator. This is representative of the rotation principle of the induction motor which is the same as that of the synchronous motor. However, the synchronous torque in the induction motor does not mean the synchronization with the rotor. Further, active load current in the secondary windings is proportional to a mechanical output.
There is a difference in the rotation speed between the equivalent DC current and the rotor. Since a rotary field which is rotated in proportional to the slip frequency is necessary for the rotor, polyphase winding is employed for the secondary winding so that the rotary field for the rotor rotates in proportional to the slip frequency. The synchronous torque generated in the induction motor is always equilibrium to the mechanical load torque of the rotor. When the load torque is increased, the slip increases so that the secondary voltage and secondary reactive current are increased. As a result, the synchronous torque is increased and, therefore, there is no stepping out. However, the rotation speed is decreased because of the increase of slip.
If the rotor of the induction motor is forced to be rotated by a prime mover, the direction of slip becomes inverted. As a result, the lead angle is generated in the rotor so that the induction motor acts as an induction generator. In this case, the lead angle corresponds to that of a synchronous generator and the same power generation can be achieved. The rotary field in the induction generator is generated using an AC power source. Accordingly, the AC power source cannot be disconnected. The generated power is controlled by changing the load of secondary winding. The torque of a prime mover changes the lead angle to change the generated power.
A selsyn motor is composed of a transmitter and receiver which have the same structure and are connected to each other in parallel. The rotor windings of the transmitter and receiver are connected to a common single-phase AC power source such that one set of AC main magnetic fluxes are generated by magnetizing currents, respectively. Generally, the stator windings are each composed of two- or three-phase winding and are connected between the transmitter and the receiver such that parallel circuits are formed in correspondence with each of phases.
It is assumed that the numbers of turns of two-phase stator windings a and b and a single-phase rotor winding c in the transmitter and receiver are equal to each other. Further, it is assumed that a single-phase AC power source outputs a constant voltage e of a sine wave with a constant frequency. In the transmitter and receiver, alternate main magnetic fluxes .phi. having the same sine wave form are generated by the c windings, respectively. In this case, if there is no voltage difference between the induced voltages by a and b, the rotors do not rotate. Therefore, the selsyn motor corresponds to two equivalent single-phase transformers. In this case, assuming that the induced voltages are e.sub.a1, and e.sub.b1 because the secondary windings correspond to a and b. In the equivalent single-phase transformers, e.sub.a1 and e.sub.b1 are equal to each other so that the direction does not change.
The transmitter has inherent characteristics. If the rotor of the transmitter is continuously rotated with a predetermined rotation speed by external force, a magnetic flux .phi. changes interlinkage angles with the secondary windings a and b because of the rotation of magnetic flux .phi. so that the induced voltages e.sub.a and e.sub.b are modulated in amplitude. Therefore, the amplitudes of induced voltages e.sub.a and e.sub.b change in proportional to the rotation speed. When the rotation speed is sufficiently small in relative to the frequency of AC power source, the amplitude can be represented by envelope lines of sine waves symmetric with respect to a center line as an outside profile line when a drum type body is cut along a center axis.
FIGS. 1A to 1C show the waveform of e for the waveforms of induced voltages e.sub.a and e.sub.b. The maximum amplitudes are equal to each other. Voltages e.sub.a1 and e.sub.b1 are modulated in amplitude because of the rotation of rotor to change into the voltages e.sub.a and e.sub.b, respectively. Thus, the AC power source voltage e functions as a single-phase carrier signal voltage. There is a phase difference between the voltages e.sub.a and e.sub.b, which indicates that the voltages e.sub.a and e.sub.b are two-phase modulated in amplitude. Expressing two-phase modulating voltages as e.sub.a2 and e.sub.b2, the voltages e.sub.a1 and e.sub.b1 are modulated in amplitude by the modulating voltages e.sub.a2 and e.sub.b2, respectively. The voltages e.sub.a and e.sub.b correspond to the waveforms obtained by synthesizing e.sub.a1 and e.sub.b2, and e.sub.b1 and e.sub.b2, respectively. Magnetizing current produced from the single-phase carrier power source generates alternate main magnetic flux .phi. which has a predetermined constant amplitude and alternates in synchronous with the carrier frequency. The two-phase modulating voltages e.sub.a2 and e.sub.b2 contained in the voltages e.sub.a and e.sub.b function to alternately deliver the alternate main magnetic flux .phi. to the secondary windings a and b in accordance with the modulating frequency. However, there is no change in the magnetizing current and alternate main magnetic flux. Further, reactive power supplied from the power source passes through the transmitter to the receiver. As a result, the transmitter and receiver are synchronously rotated. The alternate main magnetic flux .phi. inverts the direction of interlinkage with the winding a and b for every half of each rotation of the rotor. Therefore, the polarities of induced voltages e.sub.a and e.sub.b are inverted for every half wave. The polarity and phase of the carrier voltage e.sub.a1 and e.sub.b1 are also inverted repeatedly at the same time as the inversion of polarity. FIGS. 1A to 1C show the change of voltages e.sub.a and e.sub.b in amplitude and phase inversion of voltage e.sub.b2 and e.sub.b2.
The transmitter can be regarded as a type of mechanical modulator which equips all the above conditions. It has been described that any modulator is necessary for a modulation control type new AC machine according to the present invention. This mechanical modulator can be employed for the purpose. In the selsyn motor, if the transmitter is rotated in relative to the receiver, non-equilibrium voltage is generated because of non-equilibration between the rotors in angle so that e.sub.a and e.sub.b are not cancelled in each phase. As a result, the wave height of non-equilibrium voltage changes in the drum type waveform manner so that reactive circulating current reversely proportional to leakage reactance flows between the two parallel circuits. If the reactive circulating currents are synthesized in a vector manner, the synthesized reactive circulating current and the alternate main magnetic flux generates synchronous torque for removing the non-equilibrium angle by electromagnetic force. This is the operation principle of the selsyn motor.
The non-equilibrium voltage changes based on the angle difference between the rotors of transmitter and receiver. Since the non-equilibrium voltage is small compared to the voltages e.sub.a and e.sub.b, the change of synchronous torque is also small. The rotation speed is proportional to the modulation frequency but has no relation to the AC power source frequency. In the two parallel circuits, the products of the synthesized active circulating current and the voltages e.sub.a and e.sub.b represent the powers which act on the transmitter and receiver, respectively. However, they are cancelled because the directions of them are opposite to each other. Therefore, the common single-phase power supply does not operate to supply the power and active current. Actually, the selsyn motor has the inherent characteristic that any mechanical output cannot be increased because the power and active current is not supplied. Generally, the alternate main magnetic flux is obtained by synthesizing a positive phase rotary magnetic field and a reverse phase rotary magnetic field which have the same rotation speed and the same amplitude and rotates in directions opposite to each other. This is conventional technical common sense.
If one phase of two-phase primary windings of a two-phase induction motor is excited by a single-phase AC power source, this corresponds to a single-phase induction motor in which an alternate main magnetic flux is generated. However, in this case, the rotor does not rotate because there is no start torque. If the rotor is once rotated by external force, the rotor continuously rotates based on spontaneous torque. At this time, the external force may be removed. This is because a reactance difference between the positive phase reactance and the reverse phase reactance is generated because of the rotation so that the rotor is continuously rotated based on the difference between positive phase torque and reverse phase torque. The structure of receiver is similar to this and the spontaneous torque prevents the rotation operation of selsyn motor. If the spontaneous torque is increased to be greater than the synchronous torque, the receiver becomes unstable because of the spontaneous torque. The rotation speed of the selsyn motor is limited and the rotation speed cannot be increased. The single-phase rotor winding of the receiver corresponds to the DC field winding of the two-phase synchronous motor and they have the same structure. In the synchronous motor, however, when the power frequency changes, the rotation speed, magnetizing current, DC main magnetic flux and synchronous torque also change. On the other hand, there is no DC magnetic flux in the receiver, the rotation speed of the receiver is controlled by the transmitter. Thus, a predetermined alternate main magnetic flux generated by the predetermined AC power source voltage has no relation to the rotation speed. In this manner, since the selsyn motor has the peculiar characteristic and mechanical output cannot be increased.
Therefore, the selsyn motor differs basically from the synchronous motor in the characteristic and operation principle.
As described above, the existing AC machines have different characteristics and drawbacks depending upon the types and there is a strong demand for a new AC machine to overcome the above drawbacks.